In the field of cancer chemotherapy, a large number of compounds have already been put to practical use as antitumor agents. However, their activities against various types of tumors are not necessarily satisfactory, and the problems of tolerance of tumor cells to these antitumor agents complicate their clinical use.
Under these circumstances, the development of novel antitumor substances is invariably desired in the field of cancer therapy. Particularly, there is a need for substances which overcome the problem of tolerance to the previously existing antitumor substances and which exhibit efficacy against such types of cancers that they cannot have any sufficient effect on.
Indolopyrrolocarbazole derivatives having excellent antitumor activity are described in several patents, including U.S. Pat. Nos. 5,591,842 and 5,922,860. Among these indolopyrrolocarbazole derivatives, a compound of Formula I: has superior advantages over antitumor agents previously used in terms of activities and safety and therefore its practical use is highly expected. However, the solubility of this compound in water is not optimal.
It is therefore an object of the instant invention to provide a means for improving the solubility and stability of a clinical formulation comprising the compound of Formula I as an active ingredient.